If you have Windows XP, you definitely want to upgrade since there's known vulnerabilities that are not going to be fixed that are being exploited.
If you have Windows 7, you probably don't need to update unless you want to play a game that requires DirectX 12.
If you have Windows Vista or 8.1, you definitely should upgrade.
It's entirely possible to completely ignore the metro UI stuff, the Windows Store, etc. File exploring isn't slow at all. You'll want to change your privacy controls (spread throughout the entire options area) though.